Process of coating paper



Patented Oct. 19, 1943 PROCESS OF COATING PAPER William J. Montgomery, Hamilton, Ohio, asslgnor to The Champion Paper and Fibre Company, Hamilton, Ohio, a corporation of Ohio No Drawing. Application August 3, 1940, Serial No. 351,218

13 Claims.

This invention relates to the manufacture of coated paper, more particularly of paper coated with pigment and adhesive to enhance its value for high grade printing and lithography.

One object of the invention is to provide a method of smoothing freshly coated paper during the drying operation to thereby obtain a more effective smoothing than can be had by any of the known processes carried out after the drying has been completed.

Another object is to provide a process which may be applied to webs traveling at higher speeds than employed in the usual paper coating and drying operations and which is readily adaptable to direct connection with paper-making or other high speed machines.

Another object is to provide a coated paper having, before as well as after supercalendering, a superior smoothness and flatness not obtainable in the usual air dried papers or papers coated on the paper machine by previous methods and which, even before supercalendering, is suitable for fine half tone printing.

A further object is to provide a coated paper which can be more easily and effectively finished in a supercalender than can the usual air dried coated papers.

In carrying out the invention an aqueous coating composition comprising adhesive such as for example casein, starch and the like, and mineral pigment such as for example clay, calcium carbonate, blanc fixe, etc., is applied to a traveling web of paper, advantageously in the dryer section of a paper machine. No particular type of coating applying apparatus is required, and either a weight of coating corresponding to that desired on the finished paper or an excess over this amount may be applied.

While the coating is still in a wet and highly plastic or fluid condition it is spread out into a smooth, uniform layer by a pressing operation. This is accomplished by passing the paper between two cylindrical surfaces, with the coated side in contact with a layer of liquid, advantageously an oil film carried on one of the cylindrical surfaces. In one embodiment of the invention the liquid film is carried on a smooth, metallic, internally heated drum. The pressing surface may be a rubber covered roll. If an excess of coating composition is applied to the web, the

pressing operation may also be used to squeeze ofi the excess, which is then returned to the coater for reuse.

After the smoothing and pressing step, the coating may remain in contact with the liquid film for a longer or shorter time depending on the condition of the coating. Preferably the coating should be maintained in contact with the liquid film until the liquid component of the coating composition at the surface in contact with the liquid film has evaporated or soaked into the material. The coating may be substantially completely dried while in contact with the said film, but I prefer to remove the coated paper before complete drying has taken place and to complete the drying on drum driers such as are used in paper machines. In this way much higher speeds and consequently greater production may b obtained with the same size equipment.

In separating the paper from the liquid layer it is thought that the latter plits, part of the liquid remaining on the cylinder and part adhering to the coating. This plus the evaporation losses make it necessary to continuously replenish the supply of liquid as will be hereinafter described.

It is characteristic of my process that the coating does not come in contact with a solid surface during the aforesaid smoothing operation, but is separated from the solid smoothing surface by a layer of liquid.

The liquid film used to cover the smoothing surface should be substantially immiscible with the aqueous coating composition as otherwise difficulty may be experienced in preventing the liquid from rapidly diffusing into the coating composition, especially if the latter is present in a very fluid condition. Oils in general serve this purpose very well. Other liquids not ordinarily considered as oils in the narrow sense of the term, such as for example tributyl phosphate, xylene, turpentine, monochlorbenzene, etc., have also been found suitable. So far as I know, any liquid which will wet the backing surface and is sufilciently fluid to spread into a continuous film yet does not vaporize too rapidly at the operating temperatures (which are chosen to suit the particular conditions and are usually between and 210 degrees F.) and which will not so rapidly diffuse into the coating as to no longer be present as a film, will serve as the liquid forming surface. For obvious reasons colorless, odorless l quids are particularly desirable. Deodorized kerosene and the so-called mineral white oils, especially those of lower viscosity, have been found to give highly satisfactory results. Sulphonated vegetable oils have been found useful under certain circumstances; however it is thought that in such cases a film of unsulphonated oil deposited on the smoothing surface and immiscible with the aqueous coating, i the effective ingredient.

Liquid films of the necessary thinness (which may be of the order of one half micron) and uniformity can be applied to the solid backing surface in any suitable manner. For example, I have found that liquid may be wiped on by means of a suitable absorbent medium such as felt or wicking kept moist with the liquid. Alternatively it may be transferred to the solid surface by means of a roll covered with felt or similar absorbent medium, or with rubber, synthetic rubber, or with other composition which is suitably resistant to damage by the liquid used. I have also found that in the case of liquids which are sufficiently volatile, a. satisfactory film can be condensed on the backing surface by vaporizing the liquid, advantageously in close proximity to the surface, by the application of heat, and bringing the vapor into the contact with the solid backing surface. In order to replace liquid lost by evaporation or carried away with the coated paper, the application is normally a continuous operation.

In some cases it is desirable to supplement the film on the backing surface by incorporating some of the liquid in the coating composition before it is applied to the paper. The liquid should not be emulsified too finely in the coating composition if it is to have the maximum effectiveness. Its incorporation, therefore, is advantageously the last step, or nearly the last step, in the preparation of the coating, and the coating should preferably not be unnecessarily subjected to any milling or mixing operations thereafter.

Frequently it has been found advisable to incorporate some of the liquid in the coating material and to apply some directly to the solid backing surface as well.

The process is broadly useful with a considerable variety of coatings as used in the art and is not to be considered as requiring the use of any specific type of coating composition. It is advantageous in many cases to add to the coating composition a small amount (of the order of several percent) of zinc or aluminum stearate. These materials resist wetting in a manner similar to oil and seem to increase the effectiveness of the oil.

In order to obtain the best results it is desirable that the backing surface which carries the layer of liquid should be smooth, and it may be polished. Any of the usual metals may be used. It is advantageous that the surface be corrosion resistant so that it will retain its smoothness and not stain the coating. A chromium surface is particularly satisfactory. In case the surface is not readily wet with oil, the wetting and proper spreading of the oil film may be facilitated by first coating the surface with magnesium oleate or other insoluble soap of an alkaline earth metal. The backing surface may be a cylindrical drum or roll or it may be in the form of an endless metal belt.

The invention may be carried out accordingto a number of modifications in details. In the first place, the coating may be applied to the paper substantially simultaneously with the pressing of the paper against the oiled surface. In this case the press roll serves the double purpose of limiting the weight of coating applied and of smoothing the coating. In this embodiment of the invention it is generally desirable to use a coating and stock of a character to promote absorption of the liquid component of the coating composition into the paper. The coating which passes through the nip between the press roll and the oiled cylinder will usually carry some fiuid coating on the surface of the paper which will, however, be substantially absorbed by the time the paper is removed from the backing cylinder.

As an alternate procedure, a certain time may be permitted to elapse between the initial application of excess coating to the web and the pressing of the coated surface against the oiled cylinder. In this case also, the pressing operation serves the double purpose of limiting the weight of coating applied and of smoothing the coating. It removes some of the liquid coating, but it may also allow some to pass through on the surface of the web. Here again it is desirable that there be no significant quantity of liquid coating composition on the surface of the coating when the web breaks contact with the oil film; otherwise the coating is likely to split, some of it remaining on the cylinder where it may form an undesirable dust, while the coated paper assumes a stippled appearance and has inferior printing quality. Elimination of this liquid coating composition may be hastened by use of a heated cylinder.

The invention .also embraces the methods wherein the desired thickness or weight of coating is obtained by applying to the web the correct quantity of coating composition. In such case it is advisable that there be substantially no liquid coating composition on the paper at the time it is pressed. The elimination of the liquid coating composition may occur by the absorption of the liquid into the underlying material or by partial drying, e. g. by passing the web around a heated drying drum with the uncoated side in contact with the drum. The result is that at the time this coated paper reaches the pressing roll there is substantially no liquid coating present on the web and the press roll serves to smooth the coating and not to remove any excess. In those cases where the desired weight of coating is applied the smoothing operation may be accomplished by passing the web between a pair of press rolls (the roll which contacts the coating carrying a film of oil). i Or in place of this momentary contact provided by a pair of press rolls, the paper may be pressed against and carried partially around a smooth, oiled, heated dryer drum.

The following examples are given to illustrate the process:

Example 1.-A paper base coating stock consisting of bleached sulfate and bleached soda pulps and containing a moderate amount of mineral filler and traveling at a speed of 480 feet per minute was coated with an excess of coating composition. The coating composition comprised parts finely precipitated calcium carbonate, and 12 parts casein dissolved in ammonia water. These ingredients were thoroughly mixed in the presence of enough water to give a solids content of about 54.5%. Thereafter about 4% deodorized kerosene, 0.6% tributyl phosphate and 2 of zinc stearate (based on total weight of the coating composition) were incorporated. It was applied to the paper by means of a roll dipping into a pan continuously supplied with the coating composition. After passing approximately one and a half feet from the coating applying roll the paper was firmly pressed against a six-foot diameter mirror polished, chromium plated drum internally heated by steam to a temperature of 167 degrees F. The press roll used to press the coated side against the drum was a six-inch diassrpea ameter rubber covered roll under a pressure of about 100 pounds per linear inch. The press roll served to spread and smooth the coating and to limit the amount of coating to about pounds (dry weight) per thousand square feet. The excess coating formed a pool in the nip and dropped back into a pan and flowed back to the coating applying pan. Under these conditions considerable of the coating on the paper passing under the press roll was in the form of a fluid coating. A thin film of deodorized kerosene was continuously applied to the face of the drum by oil resistant synthetic rubber rolls, the amount of oil used corresponding to about /3 pound per thousand square feet of coated paper surface. After passing about three quarters around the periphery of the drum the paper passed under a take-off roll and was removed from the drum while still wet and was subsequently dried to a normal moisture content of about 3 or 4%.

Before any calendering or supercalendering the product was a smooth coated one side paper having an extremely low gloss (Bausch & Lomb Glossmeter reading of 11.5) and even in this condition could be printed satisfactorily in half tones with a 133 line screen. Supercalendering developed a high gloss, still further smoothed the surface and enhanced the printing qualities.

Example 2.In this case 30% of the calcium carbonate of the above composition was replaced with a high finishing clay. Other conditions were the same except the paper speed was 400 feet per minute, the drum temperature was 150 degrees F., total solids in the coating were 50%, coating weight was 3 lbs. per 1000 sq. ft. and oil was distilled onto the surface of the drum in place of being applied by rolls. The uncalendered product had a very low gloss (22) but was a smooth coated paper acceptable for fine half tone printing and after supercalendering had a gloss of 67 and was highly satisfactory for printing.

Example 3.In this case using substantially the same coating composition as in Example 1 the paper base chosen was more absorbent to the coating composition and traveled at a slower rate (250 feet per minute). Approximately ten times as much time was given for the coating to be in contact with the paper before being pressed against the heated chromium plated drum. This allowed the absorption of a substantial part of the liquid content of the coating composition into the paper, leaving on the surface of the paper a firm layer of the solid components of the coating composition. Under these conditions it was possible to remove the paper from the oil film on the chromium plated surface substantially immediately after it passed under the rubber covered press roll. The excess fluid coating was pressed off the sheet and returned for reuse, leaving a coating on the web of about 4 lbs. per 1000 sq. ft. (dry weight). An oil film was applied to the chromium surface by means of a roll.

Example 4.-A usual grade of coating stock was coated with a coating composition containing 100 parts of china clay and 13 parts of casein dissolved with soda ash, and 113 parts of water. No oil or other water-repelling agent was used in the coating composition. The coating composition was heated to a temperature of 120 degrees F. and an excess was applied to the paper. While there was still a large amount of fluid coating on the paper it was pressed against an oiled chromium surface heated to a temperature of about 160 degrees F. The excess coating was removed by the press roll, leaving on the sheet about 3 lbs. (dry weight) per 1000 sq. ft. The press roll used was a 5 inch diameter rubber covered roll exerting a pressure of about lbs. per linear inch. This coating remained in contact with the oiled chromium surface several seconds. after which it was removed and finally dried.

This paper after supercalendering had a gloss of 53 as compared with a finish of only 40 on paper which had been coated with the same coating composition, air dried and supercalendered but without having been pressed against the oiled chromium surface as described. Along with the higher finish there was also an improvement in the printing qualities.

E wample 5.-A coating composition comprising parts finely precipitated calcium carbonate, 12 parts casein dissolved in ammonia. water, 4 parts zinc stearate, 6 parts deodorized kerosene and 116 parts of water was applied to a good grade of coating stock in excess of the quantity desired on the finished paper. The paper was thereafter pressed against a polished steel surface heated to a temperature of about 160 degrees F. and carrying a thin film of deodorized kerosene. Pressing was accomplished by a 5 inch rubber covered press roll which served both to smooth the coating and to remove excess fluid coating. After pressing, the paper was substantially immediately removed from the oiled steel surface. A very satisfactory coated paper carrying about 5 lbs. (dry weight) per 1000 sq. ft. was obtained after drying the paper.

Example 6.In another case a good grade of coating stock was coated with a coating composition consisting of 100 parts of calcium carbonate, 17 parts of starch, 4 parts aluminum stearate, 6 parts deodorized kerosene and 120 parts of water. Otherwise the conditions were substantially similar to those given in Example 5 except that an oiled chromium surface was used. The resulting paper after supercalendering had a gloss of 50 and printed satisfactorily.

Example 7.--A coating composition comprising 100 parts china clay, 17 parts starch, 2 /2 parts pine oil and 120 parts of water was used. The coating conditions were similar to those mentioned under Example 4. The finished paper after supercalendering had a gloss of 38 as compared with a gloss of 18 on a coated paper similar in every respect except that the freshly coated paper had not been pressed against the oiled chromium surface. There was also a definite improvement in the printability of this paper.

Example 8.A usual grade of coating stock was coated with a coating composition comprising 50 parts china clay. 50 parts of finely precipitated calcium carbonate, 15 parts casein dissolved with soda ash and parts of water. This coating contained no oil or stearate. A coating weight (dry basis) of 4 lbs, per 1000 sq. ft. was applied to the paper by means of a blade coater. The coating was partially set by contacting the back (uncoated) side of the paper with a steam heated drum so that by the time it underwent the pressing operation there was substantially no fluid coating apparent on the paper. It was then pressed against a heated degrees F.) oiled, polished steel surface and substantially immediately after pressing was removed and dried. This paper had a gloss of 17 before calendering and after supercalendei'ing had a gloss of 43. In comparison with these results a paper carrying the same coating composition but not pressed had a gloss of 10 uncalendered and after supercalendering in exactly the same manner had a gloss of 20. That is to say, the supercalendering operation increased the gloss of the coating dried in the usual manner by points or 100%, whereas it increased the gloss of the coating finished according to the present invention by 26 points or over 150%, clearly showing how use of this process increases the ease of finishing in a supercalender. From the printing standpoint and in general this paper had been definitely improved by the pressing operation described.

The invention has particular advantages when carried out in connection with a paper machine operation, particularly when the coating is one that can be applied at the normal operating speeds of the paper machine. The necessary apparatus for carrying out the process, e. g., a pan and applying roller for the coating composition, and a heated drum with pressure mller, can be readily installed in the dryer section of the paper machine at a point where the number of drying drums beyond the smoothing cylinder are sufficient to properly dry the coated paper before it reaches the calender rolls. Instead of incorporating the coating apparatus in the paper machine, the calendered or uncalendered web maybe led from the dry end of the paper machine directly to the coating apparatus which will of course be synchronized with it. The process may also be carried out as a separate paper coating process.

Coated paper (either one or two side coated) made according to the present invention has a smoothness and flatness even before any supercalendering, sufficient to serve as a base for high grade half tone printing. In this respect it is far superior to the usual grades of coatedpaper which are air dried on festoons. The smoothness can, however, be still further increased by supercalendering, and it is much more easily and effectively finished in the supercalender than are the usual types of coated paper.

I claim:

1. Process of making coated paper having a smooth but non-cast surface, which comprises applying to a web of paper aqueous coating composition comprising adhesive and mineral filler, maintaining on a solid smoothing surface a substantially unbroken liquid film of a character not to be wholly absorbed upon contact with said coating composition, pressing the coated side of the paper while the coating is still wet, against the liquid film on said smoothing surface, and removing the web from the smoothing surface after the coating composition is no longer liquid, said liquid film being maintained of a thickness suflicient to so reduce the adhesion between the applied coating and the underlying surface that the paper web may contract freely during drying.

2. Process of making coated paper having a smooth but non-cast surface which comprises applying aqueous coating composition comprising adhesive and mineral filler to a web of paper, pressing the wet coated side of the paper against an unbroken liquid film which is substantially immiscible in the aqueous coating composition and which is carried on a solid surface, removing the paper from contact with said liquid film after the coating is no longer in a liquid condition, and then drying the coated paper.

3. Process of making coated paper having a smooth but non-cast surface, which comprises applying liquid aqueous coating composition in excess of that desired on the finished paper to a web of paper, pressing the coated side of the paper while the coating is still wet, against a liquid substantially insoluble in the vehicle of the coating composition, carried on a solid surface and maintained as an unbroken liquid film thereon, to smooth the coating and to remove excess coating composition, and removing the web from contact with said liquid after the coating composition is no longer in a liquid condition.

4. Process of making coated paper having a smooth but non-cast surface, which comprises applying an excess of aqueous coating composi-- tion comprising adhesive and mineral filler to a web of paper and then after the absorption of s'uiicient water to forms. semisolid layer substantially corresponding to the weight of coating desired on the finished paper, pressing the coated side of the paper against an unbroken liquid film, which is substantially immiscible with said aqueous coating composition and which is carried on a solid surface, said pressing operation serving to remove the excess fluid coating composition from above said semisolid layer, and to smooth the surface of said semisolid layer, and thereafter removing the web from contact with said liquid film.

5. Process of making coated paper having a non-cast surface, which comprises applying to a traveling web of paper an excess of aqueous coating composition comprising adhesive and mineral pigment, smoothing the coating and removing excess coating composition by pressing the freshly coated side of the paper against an unbroken layer of liquid substantially immiscible with water, carried on a rotating, heated cylin-' drical surface, partially drying the coating and removing the paper from said surface after the coating is substantially all in the form of a moist firm layer and thereafter drying the coated paper.

6. Process of making coated paper having a non-cast surface, which comprises applying to a web of paper in the drier section of a paper machine aqueous coating composition comprising adhesive and mineral filler in excess of the amount desired on the finished paper, smoothing the coating and removing the excess coating by pressing the freshly coated side of the paper against a heated,-rotating surface upon which is maintained a substantially unbroken liquid oil film, continuously applying a-film of oil to said surface, partially drying the coating while in contact with said oil film, removing the paper from said surface after the coating has partially dried, and thereafter completing the drying of the paper on heated dryer drums.

'7. Process of making coated paper which comprises applying to a traveling web of paper aqueous coating composition comprising adhesive and mineral filler in amount corresponding to that desired on the finished paper, partially drying said coating until there is substantially no fluid coating remaining, smoothing the coating by pressing the freshly coated side of the paper against a smooth cylindrical surface of a rotating cylinder, maintaining on the surface of said against a substantially unbroken liquid layer comprising said water-immiscible liquid carried on a solid surface of a character to be wetted by said water-immiscible liquid and separating the web from said liquid layer after the coating composition is no longer liquid.

9. Process of making coated paper having a non-cast surface, which comprises applying fluid aqueous coating composition comprising adhesive, mineral filler and an insoluble soap to a web of paper, pressing the coated side of the paper while the coating is wet, against a solid surface carrying thereon a liquid film which is substantially immiscible with said fluid coating composition, and separating the web from said surface after the coating composition has at least partially dried to form a firm layer,

10. Process of making coated paper which comprises applying aqueous coating composition comprising adhesive and mineral filler and containing an oil, to a web of paper; pressing the wet coated side of the paper against an oil film which is carried on a solid surface, and which is maintained thereon by adsorption of oil from the coating composition; removing the paper from said oil film after the coating has at least partially solidified; and drying the coated paper to the desired degree of dryness.

11. Process of making coated paper having a non-cast surface, which comprises applying to a web of paper, an aqueous coating composition comprising adhesive, mineral filler, and an insoluble soap chosen from the class consisting of zinc stearate and aluminum stearate, pressing the coated side of the paper while the coating is still wet, against a solid surface carrying a liquid film which is substantially immiscible with said coating composition, and removing the web from said surface after the coating composition has at least partially dried to form a firm layer.

12. Process of making coated paper having a smooth but non-cast surface, which comprises applying aqueous coating composition comprising adhesive and mineral filler and containing an oil, to one side of a web of paper; pressing the wet coated side of the paper against a substantially unbroken liquid oil film which is carried on a solid surface and which is maintainedthereon by adsorption of oil from the coating composition; removing the coated paper from the oil film on said solid surface when the coating thereon is no longer liquid, and drying the coated paper to the desired degree of dryness, said liquid film being maintained of a thickness suillcient to so reduce the adhesion between the applied coating and the underlying surface that the paper web may contract freely during drying.

13. Process of making coated paper which comprises applying an aqueous coating composition comprising adhesive, mineral filler and a liquid substantially immiscible with water to a web of paper, pressing the wet coated side of the paper against a substantially unbroken liquid layer comprising said water-immiscible liquid maintained on a solid surface of a character to be wetted by said water-immiscible liquid by adsorption of said liquid from said coating composition, and separating the web from said liquid layer after the coating composition is no longer liquid.

-WIILIAM J. MONTGOMERY. 

